Drying process and apparatus for ceramic greenware

ABSTRACT

A method and system for drying a honeycomb structure having an original liquid vehicle content includes exposing the honeycomb structure to a first electromagnetic radiation source until the liquid vehicle content is between about 20% and about 60% of the original liquid vehicle content, exposing the honeycomb structure to a second electromagnetic radiation source different from the first electromagnetic radiation source until the liquid vehicle content is between about 0% and about 30% of the original liquid vehicle content, and exposing the honeycomb structure to convection heating until the liquid vehicle content is between about 0% and about 30% of the original liquid vehicle content.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. provisionalapplication No. 61/130,370, filed on May 30, 2008.

FIELD

This disclosure relates to a method for drying ceramic greenware, and inparticular, to a method for drying a honeycomb structure in a mannerthat reduces the amount of time required for sufficient drying thereofwhile simultaneously maintaining and reducing the amount of fissuresproduced within those structural bodies as compared to previouslyemployed methods requiring longer relative drying times.

BACKGROUND

In an attempt to reduce atmospheric pollution, many countries areimposing increasingly stringent limits on the composition of the exhaustgases produced by internal combustion engines and released into theatmosphere. The primary harmful substances from internal combustionengines include hydrocarbons, carbon monoxide, nitrogen oxides (NOx) andparticulate matter. Heretofore, many methods have been proposed in anattempt to reduce or minimize the quantity of such substances present inthe exhaust gases emitted into the environment.

The use of honeycomb structures as filters for removing particulates(e.g., soot) from engine exhaust gases, and as substrates for supportingcatalytic materials for purifying engine exhaust gases is known. Aparticulate filter body may be, for example, a honeycomb article havinga matrix of intersecting thin, porous walls that extend across andbetween its two opposing open end faces and form a large number ofadjoining hollow passages, or cells, which also extend between and areopen at the end faces. To form a filter, a first subset of cells isclosed at one end face, and the remaining cells are closed at the otherend face. A contaminated gas is brought under pressure to one face (the“inlet face”) and enters the filter body via the cells that are open atthe inlet face (the “inlet cells”). Because the inlet cells are sealedat the remaining end face (the “outlet face”) of the body, thecontaminated gas is forced through the thin, porous walls into adjoiningcells that are sealed at the inlet face and open at the opposing outletface of the filter body (the “outlet cells”). The solid particulatecontaminants in the exhaust gas (such as soot), which are too large topass through the porous openings in the walls, are left behind, andcleaned exhaust gas exits the outlet face of the filter body through theoutlet cells.

A substrate for supporting catalytic materials may similarly be ahoneycomb structure having a matrix of intersecting walls that extendacross and between its two opposing open end faces and form a largenumber of adjoining hollow passages, or cells, which also extend betweenand are open at the end faces. The walls are coated with a catalyticmaterial selected to reduce the amount of carbon monoxide (CO), nitrogenoxides (NOx), and/or unburned hydrocarbons (HC) in the exhaust gas asthe exhaust gas passes through the cells. These honeycomb structures(i.e., filters and substrates) may have transverse cross-sectionalcellular densities of approximately 1/10 to 100 cells or more per squarecentimeter.

Such honeycomb structures are typically formed by an extrusion processwhere a material is extruded in a green (uncured) body before the greenbody is fired to form the final ceramic material of the honeycombstructure. The extruded green bodies can be any size or shape and haverelatively low mechanical strength. As used herein, ceramic greenware,or more briefly greenware, refers to bodies comprised of ceramic-formingcomponents that, upon firing at high temperature, form ceramic bodies.The greenware may include ceramic-forming precursor components, ceramiccomponents, and mixtures of various ceramic-forming components andceramic components. The various components can be mixed together with aliquid vehicle such as, for example, water or glycol. Immediately afterextrusion, the greenware possesses some given liquid vehicle content,such as a water or glycol content, at least some of which must beremoved, i.e., the greenware must be dried, prior to firing at hightemperature.

The drying process must be carried out in a manner that does not causedefects the greenware, such as shape change, cracks, fissures, and thelike. Such defects tend to occur when the greenware is overheated duringthe drying process.

SUMMARY

One aspect is a method for drying a honeycomb structure comprising thesteps of providing a honeycomb structure having an original liquidvehicle content, and exposing the honeycomb structure to a firstelectromagnetic radiation until the liquid vehicle content is betweenabout 20% and about 60% of the original liquid vehicle content. Themethod further comprises exposing the honeycomb structure to a secondelectromagnetic radiation different from the first electromagneticradiation until the liquid vehicle content is between about 0% and about30% of the original liquid vehicle content, and exposing the honeycombstructure to convection heating until the liquid vehicle content isbetween about 0% and about 10% of the original liquid vehicle content.

Another aspect includes a system for drying a ceramic greenware thatincludes a liquid vehicle. In one embodiment, the system comprises amicrowave drying center having a microwave generating apparatus adaptedto dry the greenware by subjecting the greenware to microwaves, a radiofrequency (RF) drying center having an RF generating apparatus adaptedto dry the greenware by subjecting the greenware to RF waves, aconvection heating center having a convection heating apparatus adaptedto dry the greenware by subjecting the greenware to convection heating,and transport means configured to transport the greenware between themicrowave drying center and the RF drying center, and between the RFdrying center and the convection heating center.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicingembodiments as described herein, including the detailed description thatfollows, the claims as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments that are intendedto provide an overview of framework for understanding the nature andcharacter of the invention as it is claimed. The accompanying drawingsare included to provide a further understanding, and are incorporatedinto and constitute a part of the specification. The drawings illustratevarious embodiments, and together with the description served to explainthe principals and operations of the invention as it is claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an example honeycombstructural body having a plurality of open-ended,longitudinally-extending channels;

FIG. 2A is a flow chart illustrating one process for manufacturing ahoneycomb structural body;

FIG. 2B is a flow chart illustrating an example ceramic greenware dryingmethod; and

FIG. 3 is a schematic illustration of an example greenware formingsystem including a greenware drying system utilized to manufacture ahoneycomb structural body.

DETAILED DESCRIPTION

Reference is now made in detail to embodiments which are illustrated inthe accompanying drawings. Whenever possible, the same referencenumerals and symbols are used throughout the drawings to refer to thesame or like parts.

Honeycomb structures used for solid particulate filtering, catalyticsubstrates, and other applications may be formed from a variety ofporous materials including, for example, ceramics, glass-ceramics,ceramic-forming components, glasses, metals, cements, resins or organicpolymers, papers, or textile fabrics (with or without fillers, etc.),and various combinations thereof. Honeycomb structures having uniformlythin, porous and interconnected walls for solid particulate filteringapplications are preferably fabricated from plastically formable andsinterable substances that yield a porous, sintered material after beingfired to affect their sintering, such as metallic powders, ceramics,glass-ceramics, cements, and other ceramic-bases mixtures. According tocertain embodiments, honeycomb structures may be formed from a porousceramic material, such as cordierite, silicon carbide, or aluminumtitanate. Cordierite is a ceramic composition (2MgO—2Al₂O₃—5SiO₂) havinga very low thermal expansion coefficient, which makes the materialresistant to extreme thermal cycling. Cordierite also exhibits hightemperature resistance (1200° C.) and good mechanical strength.

The batch raw materials used in the method of the present disclosureinclude sources of silica, alumina, titania, and at least one alkalineearth metal. The alkaline earth metal is preferably selected from thegroup of strontium, barium, calcium, and combinations of these. The rawmaterials may also include, in combination with those listed above, ironoxide. Most preferably, the batch of inorganic raw materials, asexpressed on a weight percent oxide basis, includes 40-65% Al₂O₃; 25-40%TiO₂; 3-12% SiO₂; and 2-10% of an alkaline earth metal oxide selectedfrom the group consisting of SrO, CaO, BaO, and combinations thereof.

To this mixture of components of inorganic raw material components andrare earth metal oxide it is further added processing aids selected fromthe group of organic and/or organometallic binders, lubricants,plasticizers, pore formers, and aqueous or non-aqueous solvents to forma preferably homogenous and plastic mixture that can be shaped bymolding or extrusion. The pore former, such as graphite, starch orpolyethylene may optionally be added in order to increase the porosityof the final product. The weight percent of the processing aids arecomputed as follows: 100×[(processing aid)/(total wt. of inorganic rawmaterials)].

As an example, FIG. 1 illustrates a solid particulate filter body 10.The filter body 10 includes a honeycomb structure 12 formed by a matrixof intersecting, thin, porous walls 14 surrounded by an outer wall 15,which in the illustrated example is provided in a circularcross-sectional configuration. The walls 14 extend across and between afirst end 13 that includes a first end face 18, and a second end 17 thatincludes an opposing opposite end face 20, and form a large number ofadjoining hollow passages or cell channels 22 which also extend betweenand are open at the end faces 18, 20 of the honeycomb structure 12. Thewalls 14 have porosity suitable for the intended application (e.g., afilter or substrate) of the honeycomb structure 12, and may have eithera uniform thickness or a non-uniform thickness, depending upon theintended application. The thickness and spacing of the walls 14 areselected to provide a density of the cell channels 22 as is desired forthe intended application. In some applications, density of the cellchannels 22 is in the range of 100-900 cells per square inch, althoughcell densities lower and higher than that range may also be used. Eachcell channel 22 may have a square cross section or may have other cellgeometry, e.g., circular, rectangular, triangular, hexagonal, etc.Accordingly, as used in this disclosure, the term “honeycomb structure”is intended to include structures having a generally honeycomb structureand is not limited to a particular cell geometry.

To form some embodiments of a filter, one end of each of the cellchannels 22 is sealed, a first subset of the cells being sealed at thefirst end face 18, and a second subset of the cell channels 22 beingsealed at the second end face 20. In an example cell structure, eachinlet cell channel is bordered on one or more sides by outlet cellchannels and vice versa.

In operation, contaminated fluid (e.g., exhaust gas from a combustionengine) is brought under pressure to an inlet face (i.e., the first endface 18), and enters the resultant filter via the cell channels 22 whichhave an open end at the given inlet face. Because the cell channels 22are sealed at the opposite end face, i.e., the outlet face of the body(i.e., the second end face 20), the contaminated fluid is forced throughthe thin porous walls 14 into adjoining cell channels 22 which aresealed at the inlet face 18 and open to the outlet face 20. The solidparticulate contaminates in the fluid, which are too large to passthrough the porous openings in the cell walls 14, are left behind and acleansed fluid exits the filter 10 through the outlet cell channels 22.

Ceramic bodies of honeycomb configuration, or ceramic honeycombstructures, i.e., cellular ceramic bodies, are constructed by preparinga ceramic green body through mixing of, for example, combinations ofceramic precursor materials, ceramic materials, temporary binders,liquid vehicles (including, but not limited to water, glycol, and thelike), and various carbonaceous materials, including extrusion andforming aids, to form a plasticized batch, forming the body into ahoneycomb-shaped greenware body through extrusion of the plasticizedbatch, drying the greenware body, and finally firing the greenware bodyin a firing furnace at a predetermined temperature.

Referring to FIG. 2, an example method for manufacturing the honeycombstructure 12 described above includes the steps of batch mixing 28 aceramic solution used to form the honeycomb structure 12, extruding 30the ceramic solution through die sets thereby forming a greenwarehoneycomb structure, cutting 32 the greenware into a particular length,and drying 34 of the greenware to form a hardened honeycomb structure.As known in the art, the extrusion operation can be done, for example,using a hydraulic ram extruder, or a two stage de-airing single augerextruder, or a twin screw mixer with a die assembly attached to thedischarge end. In the latter mentioned extrusion operation, the properscrew elements are chosen according to material and other processconditions in order to build up sufficient pressure to force the batchmaterial through the die. The extrusion can occur in a vertical plane ora horizontal plane.

Optionally, as illustrated in FIG. 2A, the method may further includeone or more additional steps such as cutting 36 the hardened honeycombstructure to provide finished end faces, removing the dust 38 createdduring the cutting process 36, masking 40 the end faces of the honeycombstructure, plugging 42 certain cell channels of the honeycomb structure(i.e., when forming a filter from the honeycomb structure), firing 44 ofthe honeycomb structure, and machining 46 an outer skin of the filter.The method may also optionally include testing 48 the filter andpackaging 50 the same for shipment.

After the firing step 46, the greenware transforms into a bodycomprising ceramic material, such as cordierite, and has a honeycombstructure with thin interconnecting porous walls that form parallel cellchannels longitudinally extending between end faces, as disclosed, forexample, in U.S. Pat. No. 2,884,091, U.S. Pat. No. 2,952,333, U.S. Pat.No. 3,242,649, U.S. Pat. No. 3,885,997 and U.S. Pat. No. 5,403,787 whichpatents are incorporated by reference herein in their entirety.Exemplary inorganic batch component mixtures suitable for formingcordierite-based bodies are disclosed, for example, in U.S. Pat. No.5,258,150; U.S. Pat. Pubs. Nos. 2004/0261384 and 2004/0029707; and U.S.Pat. No. RE 38,888, while U.S. Pat. No. 4,992,233 and U.S. Pat. No.5,011,529 describe honeycombs of similar cellular structure extrudedfrom batches incorporating metal powders, all of which are incorporatedby reference herein in their entirety. Other exemplary ceramic bodiescomprised of aluminum-titanate (AT) based ceramic materials arediscussed, for example in U.S. Pat. No. 7,001,861, U.S. Pat. No.6,942,713, U.S. Pat. No. 6,620,751, and U.S. Pat. No. 7,259,120, whichpatents are incorporated by reference herein in their entirety. AT-basedbodies can be used as an alternative to cordierite and silicon carbide(SiC) bodies for high-temperature applications, such as automotiveemissions control applications. The systems and methods disclosed hereinapply to any type of greenware 20 amenable to electromagnetic radiationand convection drying techniques.

With particular focus on the greenware drying step 34, it is noted thatextruded greenware contains a liquid vehicle (e.g., water, glycol andthe like) in the range of about 10-25% by weight, and that the greenwareneeds to be dried in the process of forming the final product. Microwave(MW) drying methods can quickly remove the liquid vehicle content anddry the greenware. Microwave drying works best when the wet material andthe dry material comprising the greenware have very different dielectricproperties. Specifically, MW drying works best when the wet material isa high loss material (due to the liquid vehicle component), and the drymaterial is a low loss material. In this way, as the greenware is dried,the microwave generated electromagnetic field interacts most stronglywith the wettest parts of the greenware. Concurrently, the driest partsof the greenware become substantially transparent at the microwavewavelength, thus preventing runaway heating of the greenware and thedefects resulting therefrom, most notable of which are shape change andcracking of the greenware. Runaway heating may occur with greenware inwhich both wet and dry components include high loss materials.Specifically, high loss dry material of the greenware continues toabsorb microwave energy, but no longer has the endothermic evaporativecooling provided by the evaporating liquid vehicle content to preventrunaway heating. Thus, continuing to supply microwave energy to thegreenware increases the temperature of the dry areas of the ware(typically the ends, but sometimes other areas, dependent onorientation, size, geometry, material and dryer configuration). Thosedry areas may become hot enough to start the decomposition process ofthe organics present in the greenware, which is undesirable. Indicationsof decomposition of organics in the greenware include but are notlimited to, for example, smoldering, burning of the methocel, loss ofoils in the extrudate, ignition of the oils in the extrudate, and thelike. However, if MW drying is stopped to prevent runaway heating,additional drying of the greenware may still be necessary as the moreinterior regions of the greenware may still be wet.

Runaway heating is prevented by applying multiple drying steps, withoutrequiring changes to the desired ceramic-forming composition. When fullywet, the material comprising the greenware is subjected toelectromagnetic radiation heating, while runaway heating is preventeddue to the evaporative cooling of the relatively wet greenware.Microwave drying is often used for electromagnetic radiation heating anddrying, as the equipment for MW drying is generally easier to design andcontrol with high loss materials, and arcing is easily prevented. Inaddition, the surface (i.e., skin) quality of the resultant greenwarecan be better controlled due to better atmosphere control, as well asthe “conditioning” of the greenware skin caused by the low penetrationdepth of microwaves into the greenware that causes much of the heatingof the greenware to occur on the surface. However, due to the lowpenetration depth of microwaves, when the dry material of the greenwareis also high loss, the interior of the greenware may remain cool and wetlong after the skin is dry. Accordingly, additional drying usingelectromagnetic radiation at a frequency having a greater penetrationdepth than microwaves (e.g., radio frequency radiation) is beneficiallyused for continued drying of the greenware.

As illustrated in FIG. 2B, drying step 34 of FIG. 2A is a multi-stepprocess. In one embodiment, at step 34 a the greenware is exposed to afirst electromagnetic radiation until the liquid vehicle content of thegreenware is between about 20% and about 60% of the original liquidvehicle content. At step 34 b, the greenware is exposed to a secondelectromagnetic radiation different from the first electromagneticradiation of step 34 a until the liquid vehicle content of the greenwareis between about 0% and about 30% of the original liquid vehiclecontent. At step 34 c, the greenware is exposed to convection heatinguntil the liquid vehicle content of the greenware is between about 0%and about 10% of the original liquid vehicle content. Steps 34 a through34 c are described in further detail below.

Generally, the greenware is placed on trays or supports and then sentthrough a drying system. In one embodiment of a drying system, a firstelectromagnetic drying center generates a first electromagneticradiation, such as MW radiation, that is absorbed by and heats thegreenware, a second electromagnetic drying center generates a secondelectromagnetic radiation, such as RF radiation that is absorbed by andheats the greenware, and a convection drying center heats the greenwarevia convection heating. The liquid carrier is thus removed and thegreenware dried by the progressive combination of electromagneticradiation heating and convection heating. In one embodiment, the firstelectromagnetic radiation has a first penetration depth into thegreenware, and the second electromagnetic radiation has a seconddifferent penetration depth into the greenware. In one embodiment, thesecond penetration depth is greater than the first penetration depth. Inone embodiment, the first electromagnetic radiation dries the exteriorsurface of the greenware faster than the interior of the greenware.

FIG. 3 is a schematic diagram of an exemplary greenware forming system100 that includes an extruder 102 followed by a three-step drying system104 that includes a first electromagnetic radiation dryer or dryingcenter 106 having a first electromagnetic generating apparatus 107,followed by a second electromagnetic radiation dryer or drying center108 having a second electromagnetic generating apparatus 109, which issubsequently followed by a convection heating center 110. In oneembodiment, the first electromagnetic generating apparatus 107 comprisesa microwave (MW) generating apparatus. In one embodiment, the secondelectromagnetic generating apparatus 109 comprises a radio frequency(RF) generating apparatus. The greenware or honeycomb structure 12 isshown in the form of extruded pieces supported in trays 114.

The drying system 104 has an input end 116 and an output end 118. Thegreenware 12 within the trays 114 are conveyed between the input end 116and the output end 118 by suitable transport means 120. In one example,transport means 120 comprises a conveyor system having one or moreconveyor sections, namely, an input section 122, a first central section124, a second central section 126, and an output section 128. Thegreenware 12 is conveyed by transport means 120 (e.g., a conveyorsystem) between the input end 116 and the output end 118 so as to travelsequentially through the first drying center 106 to the second dryingcenter 108, and then the convection heating center 110.

The first electromagnetic drying center 106 includes a housing 130 withan input end 132, an output end 134 and an interior 136, and the firstelectromagnetic generating apparatus 107 that, in one example, generatesmicrowave radiation. The second drying center 108 includes a housing 138with an input end 140, an output end and an interior 144, and the secondelectromagnetic generating apparatus 109 that, in one example, generatesRF radiation. In one example, RF radiation may be generated, e.g., by aparallel plate applicator as is known in the art. The convection heatingcenter 110 includes a housing 150 with an input end 152, an output end154 and an interior 156, and a convection heating source 158.

In the general operation of the drying system 110, the greenware 12extruded from the extruder 102 is placed in a corresponding tray 114 andconveyed via input conveyor section 122 to the input end 116 of dryingsystem 104. The greenware 12 is conveyed into the interior 136 of thefirst electromagnetic drying center 106 where the greenware is exposed,in one example, to microwave energy. In the illustrated example, thegreenware 12 is exposed to the first electromagnetic radiation (e.g.,microwave radiation) until the liquid vehicle content of the greenware12 is within the range of about 20% to about 60% of the original liquidvehicle content of the greenware 12 prior to entering the drying system104. Preferably, the exposure of the greenware 12 to the firstelectromagnetic radiation is suspended prior to the decomposition of anyorganic material within the greenware 12. In one embodiment, exposure ofthe greenware 12 to the first electromagnetic radiation is suspendedafter any portion of the greenware reaches or exceeds the boiling pointof the liquid vehicle. In one embodiment, exposure of the greenware 12to the first electromagnetic radiation is suspended when the temperatureof the greenware is at least 20° C. less than the temperature at whichthe organic materials therein start to decompose in the greenware 12.

In one embodiment, when the first electromagnetic radiation is microwaveradiation, the microwaves are applied with the range of between 500 MHzand 30 GHz. In one embodiment, microwaves are applied within the rangeof between 800 MHz and 3 GHz. In one embodiment, microwaves are appliedwithin the range of between 890 MHz and 920 MHz. In some embodiments,the first electromagnetic radiation is applied with power levels withinthe range of about 5 kW to about 1000 kW.

Following passage through the first drying center 106, the greenware 12is conveyed to the input end 140 of second drying center 108 via thefirst central section 124 of the transport means 120 and enters theinterior 144 where the greenware is exposed to a second electromagneticradiation (e.g., RF radiation) as it passes the second electromagneticgenerating apparatus 109. In the illustrated example, the greenware 12is exposed to the second electromagnetic radiation until the liquidvehicle content of the greenware 12 is within the range of about 0% toabout 30% of the original liquid vehicle content of the greenware 12prior to entering the drying system 104.

In one embodiment, when second electromagnetic radiation is radiofrequency radiation, radio waves are applied within the range of between2 MHz and 500 MHz. In one embodiment, radio waves are applied within therange of between 4 MHz and 50 MHz. In one embodiment, radio waves areapplied within the range of between 25 MHz and 41 MHz. In someembodiments, the second electromagnetic radiation is applied with thepower levels of within the range of about 5 kW to about 1000 kW.

Following passage through second drying center 108, the greenware 12 isconveyed to the input end 152 of the convection heating center 110 viathe central conveyor second section 126 and enters the interior 156where it is dried via a convection heating process. In one embodiment,the greenware 12 enters the convection heating center 110 attemperatures of within the range of between about 80° C. and about 150°C. In one embodiment, the greenware 12 enters the convection heatingcenter 110 at temperatures between about 20° C. below the boiling pointof the liquid vehicle and about 50° C. above of the boiling point of theliquid vehicle. In one embodiment, the greenware 12 is dried viaconvection heating until the liquid vehicle content of the greenware 12is within the range of about 0% to about 10% of the original liquidvehicle content, and more preferably within the range of about 0% toabout 2% of the original liquid vehicle content. In one embodiment, thepreceding steps of drying with first and second electromagneticradiation allows the greenware 12 to be dried to the necessary stage viaconvection heating for less than about 24 hours. In one embodiment,convection heating occurs for less than about one hour. In oneembodiment, the liquid vehicle is water and the convection heating isconducted within a temperature range of between about 80° C. and about150° C. In one embodiment, the liquid vehicle is water and theconvection heating is conducted within the range of between about 100°C. and about 120° C.

Example:

By way of example, a shaped green body was cut into logs and driedaccording to the method described herein. In one implementation, acontinuous greenware extrudate having water as a liquid vehicle was cutinto logs each having an open frontal area of about 50%, a diameter ofabout 15 cm, a length of about 30 cm, and a weight of approximately 5kg. In the first electromagnetic drying center, approximately 0.4 kWhrto 0.5 kWhr of microwave energy was applied per each log. Residencetimes within the first (MW) drying center were in the range of about 15minutes to about 20 minutes. The liquid vehicle content of the greenwarelogs at the exit of the first drying center was approximately 50%(+/−5%) of the original liquid vehicle content. The greenware was thentransported to the second electromagnetic drying center, whereapproximately 0.3 kWhr to 0.4 kWhr of radio frequency energy was appliedper log. Residence times within the second (RF) drying center were inthe range of about 10 minutes to about 20 minutes. The liquid vehiclecontent of the greenware at the exit of the second drying center wasapproximately 10% (+/−5%) of the original liquid vehicle content. Afterexiting the second drying center, the greenware was transported to aconvection oven at about 110° C. for about 45 minutes to complete thedrying cycle and obtain greenware that had a liquid vehicle content ofabout 2% to 0% of the original liquid vehicle content.

As disclosed herein, in one embodiment, partial drying of the greenwareis performed by exposing the greenware to a succession of more than oneform of electromagnetic radiation heating prior to convection heating ofthe greenware, and thereby avoids the creation of potentially damaging“hot spots” on or within the greenware. The drying system and methoddescribed herein are particularly useful for greenware that containshigh loss materials, such as graphite. In one embodiment, the successionof electromagnetic radiation heating utilizes electromagnetic radiationforms having different penetration depths in the greenware. As a result,it is beneficial to use a multiple-step drying process wherein thegreenware bodies are only partially dried using electromagneticradiation. In one embodiment, microwave radiation and RF radiation areused in succession to at least partially dry the greenware and therebyallow complete drying of the greenware via convection heating in arelatively reduced amount of time. In some embodiments, the greenwaremay be protected from uneven cooling and heating before and between eachof the drying steps through the use of plastic wrap, misting, optimizingtray materials, covers, and the like.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the described embodimentswithout departing from the spirit and scope of the claimed invention.

1-22. (canceled)
 23. A system for drying ceramic greenware that includesa liquid vehicle, the system comprising: a microwave drying centerhaving a microwave generating apparatus adapted to dry a greenware bysubjecting the greenware to microwaves; an RF drying center having an RFgenerating apparatus adapted to further dry the greenware by subjectingthe greenware to RF waves; a convection heating center having aconvection heating apparatus adapted to further dry the greenware bysubjecting the greenware to convection heat; and transport meansconfigured to transport the greenware between the microwave dryingcenter and the RF drying center, and between the RF drying center andthe convection heating center.
 24. The system of claim 23, wherein themicrowave drying center is separate from the RF drying center, and theRF drying center is separate from the convection heating center.
 25. Thesystem of claim 23, wherein the microwave drying center is configured toremove between about 40% to about 80% of an original liquid vehiclecontent of the ceramic greenware, the RF drying center is configured tofurther remove between about 70% to about 100% of the original liquidvehicle content of the ceramic greenware, and the convection heatingcenter is configured to further remove between about 90% to about 100%of the original liquid vehicle content of the ceramic greenware.
 26. Thesystem of claim 23, wherein the microwave drying center is configured tosuspend subjecting the greenware to microwaves prior to decomposition oforganic materials in the honeycomb structure.
 27. The system of claim23, wherein the microwave drying center is configured to suspendsubjecting the greenware to microwaves when a temperature of thehoneycomb structure is at least 20° C. less than decompositiontemperature of organic materials in the honeycomb structure.
 27. Thesystem of claim 23, wherein the microwave drying center is configured tocontinue subjecting the greenware to microwaves until a maximumtemperature of the honeycomb structure is equal to about the boilingpoint of the liquid vehicle.
 28. The system of claim 23, wherein theconvection heating center is configured to dry the greenware bysubjecting the greenware to convection heat for a period of less thanabout 24 hours.
 29. The system of claim 23, wherein the convectionheating center is configured to dry the greenware by subjecting thegreenware to convection heat for a period of less than about 1 hour. 30.The system of claim 23, wherein the RF drying center comprises aparallel plate RF dryer.